Thermoplastic resin composition and molded articles

ABSTRACT

The objective of the present invention lies in providing a resin composition which enables water vapor permeability to be controlled over a broad range while maintaining a high gas permeability, together with molded articles or packaging materials comprising said composition, in particular a packaging material for foodstuffs like fruit and vegetables. A thermoplastic resin composition which is characterized in that a polymer (A) containing polyether chains as structural units, polyethylene (B) of density as specified in JIS K6760 of less than 0.91 g/cm 3  and a compatibilizing agent (C) are blended together in proportions by weight of A/B/C=99.5 to 0.5/0.5 to 99.5/0 to 30 (where A+B+C=100); and molded articles comprising said composition, in particular a packaging material for vegetables and fruit cultivation or foodstuffs storage.

FIELD OF THE INVENTION

The present invention relates to a thermoplastic resin composition andmoulded articles thereof. The objective lies in providing a resincomposition which, while maintaining high gas permeability, permitswater vapour permeability to be controlled over a broad range; togetherwith moulded articles or packaging materials comprising thiscomposition, in particular a packaging material for foodstuffs likefruit and vegetables.

BACKGROUND OF THE INVENTION

Thermoplastic resins such as polyolefins have been widely used hithertoin applications of various kinds on account of their ease of handlingand good balance of properties, and they are also valuable as packagingmaterials. In such circumstances, depending on the resin used, varioustypes of material can be designed and employed according to theparticular objectives, ranging from permeable films with high gaspermeability to barrier materials with low permeability, but few areendowed with high water vapour transmission. Techniques are known where,for example, a highly permeable material and a material of lowpermeability are blended or laminated. However, when used for storingfoodstuffs, there have been problems such as water vapour condensing inthe interior and water droplets adhering, so that the interior cannot beviewed, or the condensed moisture hastens deterioration of the contentsand rotting tends to occur. Moreover, there have been limits to thecontrol of gas permeability where longer term storage has beenattempted.

For the purposes of resolving these difficulties, there is known theintroduction of minute holes in the film either mechanically by means ofa needle, etc, or physicochemically by means of a laser, etc, with thepermeability being controlled by hole diameter and the density of suchholes present (see, for example, Japanese Unexamined Patent PublicationNos 47-23478, 62-148247 and 2-85181, etc). Further, there has beenproposed raising the water vapour permeability by producing extremelythin regions locally in the film without going so far as to introduceholes. However, in these methods, not only is there a considerabledifference in permeability between the fine hole regions and otherregions so that it is difficult to obtain uniformity over the film as awhole, but there are also problems such as the film strength beingweakened and the selectivity of the permeability being reduced.

On the other hand, methods have also been proposed in which any attemptsat raising the permeability of the film itself are abandoned, so thatthe gases which control metabolic action are trapped inside, and insteadan adsorbent for the harmful gases and moisture is introduced (see, forexample, Japanese Unexamined Patent Publication No. 3-14480, etc).However, such methods are troublesome and, moreover, their effect is notnecessarily adequate.

Now, compositions comprising a polyether-containing block polyamide,etc, along with a polyolefin and/or functional polyolefin are alreadyknown (see, for example, Japanese Unexamined Patent Publication No.1-163234, and European Patent Publication Nos 459862, 475963, 559284,657502 and 675167, etc). As effects, there are described moisturetransmission, high impact resilience and antistatic properties, etc.However, only a general description is given of the polyolefin, andthere is no mention of paying attention to the density thereof tocontrol the gas permeability.

DESCRIPTION OF THE INVENTION

The present invention has been made based on the discovery that, intrying to control the water vapour permeability within a desired rangewhile maintaining a high gas permeability, this objective may berealised by incorporating resin of specified density.

The present invention is a thermoplastic resin composition which ischaracterized in that a polymer (A) containing polyether chains asstructural units, polyethylene or copolymer in which polyethylene is thechief component (B) of density as specified in JIS K6760 of less than0.91 g/cm³ and a compatibilizing agent (C) are blended in proportions byweight of A/B/C=99.5 to 0.5/0.5 to 99.5/0 to 30 (where A+B+C=100);together with moulded articles or packaging materials thereof.

In the present invention, the ‘polymer (A) containing polyether chainsas structural units’ means a block copolymer in which polyoxyalkylenechains and other polymer chains are linked together, or a polymer inwhich polyoxyalkylene chains are connected together via couplingregions. Examples of the polyoxyalkylene here are polyoxyethylene,poly(1,2- and 1,3-oxypropylene), polyoxytetramethylene,polyoxyhexamethylene, ethylene oxide and propylene oxide block or randomcopolymers, ethylene oxide and tetramethylene oxide block or randomcopolymers, and the like. In particular, those with from 2 to 4 carbonsin the alkylene moiety are preferred, with polyoxyethylene being mostpreferred. The number average molecular weight of the polyoxyalkylene isfrom 200 to 6000, and preferably from 300 to 4000.

The preferred ‘polymer (A) containing polyether chains as structuralunits’ employed in the present invention is a polyether-polyamide blockcopolymer, polyether-polyester block copolymer or polyether-urethane.Amongst these, the polyether-polyamide block copolymer is especiallypreferred.

The ‘polyether-polyamide block copolymer’ used in the present inventionis a polymer in which there are linked together polyoxyalkylene chains(a) and polyamide chains (b), where the latter comprises polymer of anaminocarboxylic acid or lactam with six or more carbons, or of a salt ofdicarboxylic acid and diamine with at least six carbons. Where (a) and(b) are connected together via a dicarboxylic acid with from 4 to 20carbons, the material is generally referred to as a polyetheresteramide,and this too will be included in the invention. Here, as the‘aminocarboxylic acid or lactam with six or more carbons, or salt ofdicarboxylic acid and diamine with at least six carbons’, there ispreferably used 11-aminoundecanoic acid, 12-aminododecanoic acid,caprolactam, laurolactam, hexamethylenediamine/adipic acid salt orhexamethylene-diamine/sebacic acid salt, etc. Further, two or more typesof the aforesaid (a) and (b) components can be used together.

This polymer is produced, for example, by the method described inJapanese Examined Patent Publication No. 56-45419, etc. Specificexamples of such polymers are Pebax (Elf Atochem), ELY (EMS) andVestamid (Hòls), etc. The type and weight ratio of the polyether andpolyamide components in the block copolymer used in the presentinvention are selected according to the objectives and application. Fromthe point of view of water vapour permeability, water resistance and thehandling properties, etc, a polyether/polyamide ratio of from 4/1 to 1/4is preferred.

The ‘polyether-polyester block copolymer employed’ in the presentinvention is a polymer in which there are linked togetherpolyoxyalkylene chains (a) and polyester chains (d), where the polyesteris a polymer of a hydroxycarboxylic acid with six or more carbons, or ofa dihydroxy compound with two or more carbons and an aromaticdicarboxylic acid. Further, two or more types of these (a) and (d)components can be jointly used. This polymer is, for example, producedby the method described in U.S. Pat. No. 4,739,012. Specifically, therecan be cited Hytrel (DuPont), Pelprene P type (Toyobo) and Rekuse(Teijin). The weight ratio of the aforesaid (a) and (d) components inthe block copolymer used in the present invention will be determined bythe objectives and application. Although being thermoplastic polyesterelastomers in the same way, with polyester-polyester block copolymers(for example Pelprene S type) here is little effect.

The ‘polyether-urethane’ employed in the present invention is athermoplastic polyurethane in which the polyether is used as the softsegments. There is little effect with polyester type or caprolactonetype polyurethanes. Specifically, the polyether-urethane is normallyobtained by the reaction of an organic di-isocyanate and a polyether ofmolecular weight 500 to 6000 and, depending on the circumstances, withchain extension being conducted in the presence of catalyst.

As the isocyanate, there is preferably used tolylene diisocyanate ordiphenylmethane diisocyanate, etc, and as the polyether, there ispreferably used poly-tetramethylene glycol, polypropylene oxide orpolyoxyethylene.

In the present invention, these polyether-polyamide block copolymers,polyether-polyester block copolymers or polyether-urethanes may be usedsingly, or as mixtures thereof, or, in the case of the block copolymers,there may be used mixtures of two or more polymers having differenttypes and/or ratios of soft/hard segments in each resin, or again theremay be used blends with other resins providing that the amount is withina range such that the objectives of the present invention are realised.

In the present invention, ‘polyethylene or copolymer in whichpolyethylene is the chief component (B) of density as specified in JISK6760 of less than 0.91 g/cm³’ refers to an ethylene homopolymer, orcopolymer of ethylene and no more than 10 to 20 mol % of an α-olefinmonomer (comonomer), where said polymer or copolymer has a density asspecified in JIS K6760 of less than 0.91 g/cm³. Thus, the ordinarygeneral-purpose polyethylenes, i.e. the polyethylenes (of density 0.91g/cm³ and above) which are specified in JIS K6748 or ASTM D1248 are notincluded, and they lie outside the scope of the present invention. It isfurther preferred that the density be less than 0.90 g/cm³.

This polyethylene or copolymer in which polyethylene is the chiefcomponent is obtained by conventionally-known polymerization methods,i.e. the homopolymerization of ethylene or the copolymerization ofethylene and an α-olefin monomer (comonomer) (such as propene, butene,hexene, octene, decene or 4-methyl-1-pentene, etc) by the high pressureradical polymerization method based on oxygen or peroxide catalysts, themedium/low pressure co-ordination polymerization method based on ZieglerNatta catalysts or silica- or alumina-supported catalysts, or thepolymerization method based on ‘single site catalysts (SSC)’ withuniform points of activity as typified by Kaminsky type metallocenecatalysts. As examples of such ultra-low density polyethylene orcopolymer in which polyethylene is the chief component, there areRumitakku 9P107T or 43-1 (Toray), Eboryu SP0540 (Mitsui Petrochemical),Engage EG8150 or 8100 (Dow Chemical), and Affinity PF1140, PL1880 orFW1650 (Dow Chemical), etc.

Of these, for a given density, polyethylene or copolymer in whichpolyethylene is the chief component which is based on a single sitecatalyst is especially preferred.

In the present invention, depending on the circumstances, acompatibilizing agent (C) can be used. In the present invention‘compatibilizing agent (C)’ is a polymer employed for enhancing themiscibility of the polymer (A) containing polyether chains as structuralunits, and the polyethylene (B) of density less than 0.91 g/cm³, and itcomprises one or more than one type of polymer selected from the groupcomprising ‘polyolefins or (meth)acrylate or vinyl acetate copolymersthereof which have been grafted or co-polymerized with unsaturatedcarboxylic acids, unsaturated carboxylic acid anhydrides or unsaturatedepoxides’. Specific examples are maleic anhydride grafted polyethyleneor polypropylene, ethylene/maleic anhydride copolymer, ethylene/alkylacrylate/maleic anhydride terpolymer, ethylene/vinyl acetate/maleicanhydride terpolymer and ethylene/alkyl acrylate/glycidyl methacrylateterpolymer, etc.

The mixing proportions by weight of the polymer (A) containing polyetherchains as structural units, the polyethylene or copolymer in whichpolyethylene is the chief component (B) of density less than 0.91 g/cm³,and the compatibilizing agent (C), are A/B/C=99.5 to 0.5/0.5 to 99.5/0to 30, and preferably 95 to 5/5 to 95/1 to 20. In the storage of an itemwhich exhibits marked respiration, then the range A/B/C=99 to 55/1 to45/0 to 30, and preferably 90 to 60/10 to 40/1 to 20 is recommended, andin the storage of a comparatively readily dried item, then the rangeA/B/C=45 to 1/55 to 99/0 to 30, and preferably 40 to 5/60 to 95/1 to 20is recommended. In such circumstances, A+B+C=100 in each case. Byblending these components in the stated proportions, it is possible tocontrol the water vapour permeability over a broad range whilemaintaining a high gas permeability.

In regard to this resin composition, the respective resins can be dryblended in the specified proportions and then directly extruded toproduce a moulded article, or, prior to moulding, (A), (B) or (C) can bemelt blended and formed into pellets beforehand, or (A) and (C), or (B)and (C) can be blended beforehand and then, at the time of moulding, theblend along with (B) or (A) extruded or injection moulded to produce themoulded article. In other words, a so-called master batch method canalso be employed. The melt blending is conducted using an ordinarysingle screw or twin screw extruder, etc. The melting temperature willdepend on the types and proportions of the resins but, in general, from120 to 230° C. is employed.

In the resin composition of the present invention, there can be freelyincluded known antioxidants, thermal decomposition preventives,ultraviolet light absorbers, hydrolysis resistance improvers, colouringagents (dyes and pigments), antistatic agents, electrical conductors,crystal nucleating agents, crystallization promoters, plasticizers,ready-slip agents, lubricants, release agents, flame retardants, flameretarding auxiliaries or the like, within a range such that thecharacteristics of the invention are not lost.

The resin composition of the present invention can be used as it is toproduce extruded articles such as sheet, film or tube, etc, or injectionmoulded articles such as containers, and it can also be used afterblending with other thermoplastic resins. When producing extrudedarticles, as well as inflation, various other methods such as the T-diesystem can be employed.

EXAMPLES

Below, the present invention is explained in more specific terms bymeans of examples, but it goes without saying that the invention is notto be restricted just to these examples. Now, the various propertyvalues in the examples were measured by the following methods.

(1) Density (units: g/cm³) Measured based on JIS K6760

(2) MFI (units: g/10 minutes) Melt Flow Index Measured at a load of 2.16kg, 190° C., based on JIS K6760

(3) Intrinsic viscosity (units: g/dl) This was calculated from thesolution viscosity measured at 20° C. using a m-cresol solution of thepolymer.

(4) Moisture transmission (water vapour permeability) (units: g/m².day)Film was subjected to measurement based on JIS Z0208, under conditions B(40° C., 90% relative humidity)

(5) Gas permeability (units: ml/m².day.atm) Measurement was carried outby the pressure difference method. The specific conditions were asfollows.

device: gas permeability measurement device model GTR-10XE made byYanako Bunseki Kogyo (Co.)

test area: 15.2 cm² (44 mm diameter)

detection method: working curve system based on a gas chromatograph withTCD attached temperature,

relative humidity: 25° C., 0% RH

carrier-gas: helium 70 KPa

diffused gas: CO₂/O₂/N₂/C₂H₄ (30.0/30.0/39.12/0.88 vol %).

Further, the resins employed were as follows.

A-1: polyetheresteramide of intrinsic viscosity about 1.5, comprisingpolyoxyethylene chains (average molecular weight 1500) and polyamide 12chains (average molecular weight 1500)

A-2: polyetheresteramide of intrinsic viscosity about 1.5, comprisingpolyoxyethylene chains (average molecular weight 1500) and polyamide 12chains (average molecular weight 4500)

A-3: polyetheresteramide, comprising polyoxyethylene chains (averagemolecular weight 1500) and polyamide 6 chains (average molecular weight1500)

B-1: polyethylene of MFI 5 and density 0.87 obtained by polymerizationusing a metallocene catalyst

B-2: polyethylene of MFI 1.6 and density 0.895 obtained bypolymerization using a metallocene catalyst

B-3: polyethylene of MFI 4 and density 0.905 obtained by polymerizationusing a metallocene catalyst

B-4: polyethylene of MFI 8 and density 0.905 obtained by radicalpolymerization

B-5: polyethylene of MFI 4 and density 0.920 obtained by polymerizationusing a metallocene catalyst

B-6: polyethylene of MFI 4 and density 0.920 obtained by radicalpolymerization

C-1: ethylene/acrylate/maleic anhydride terpolymer containing 6 wt %comonomer and 3 wt % maleic anhydride

C-2: ethylene/glycidyl methacrylate copolymer containing 8 wt % ofglycidyl methacrylate.

EXAMPLES 1 to 6 Comparative Examples 1 and 2

Composition Nos 1 to 8 were obtained by mixing together resins A, B andC at a weight ratio of A/B/C=30/55/15, and then melt extruding with atwin-screw extruder set at 160-180° C., after which pellets wereproduced. Film of thickness 25μ was obtained from the pellets using asingle screw extruder fitted with a T-die, the tip of which was set at180° C. The properties of the film in each case are shown in Table 1.Where film was produced from Composition Nos 7 and 8, which employedpolyethylene of density over 0.91, although the moisture transmissionwas about the same, only poor gas permeability was obtained (ComparativeExamples 1 and 2). Further, in the case of polyethylenes of the samedensity, the material which had been polymerized by means of a singlesite catalyst showed slightly better gas permeability (contrast Examples5 and 6; Comparative Examples 1 and 2).

TABLE 1 Composition Moisture CO₂ Gas Oxygen Gas Number A B CTransmission Permeability Permeability Example 1 Composition 1 A-1 B-2C-1 140 105,000  18,100 Example 2 Composition 2 A-2 B-2 C-1 125 72,40015,200 Example 3 Composition 3 A-2 B-2 C-2 130 73,000 15,500 Example 4Composition 4 A-2 B-1 C-1 170 105,200  23,000 Example 5 Composition 5A-2 B-3 C-1  80 65,000 13,300 Example 6 Composition 6 A-2 B-4 C-1  9060,000 12,300 Comp. Ex. 1 Composition 7 A-2 B-5 C-1  70 43,700  8,800Comp. Ex. 2 Composition 8 A-2 B-6 C-1  85 41,500  8,400

EXAMPLE 7 Comparative Examples 3 to 5

Film produced from Composition Nos 2 and 8 was cut to A4 size and then,in each case, two pieces placed one on the other, and three sidesheat-sealed to produce a bag. A bunch (about 500 g) of grapes (Kyohograpes) was placed inside, and then the final edge heat-sealed. Forcomparison, bunches were packaged in the same way using a paper bag ofthe kind employed in grape cultivation and also using a commercial 25μthickness low density polyethylene bag. The bags containing the grapeswere kept for 1 month in a refrigerator at 5° C., after which they werewithdrawn and the contents checked. In the case of the paper bag, it wasfound that the stems had turned brown and individual grapes had tendedto drop away from the bunch (Comparative Example 3). Further, in thecase of the commercial polyethylene bag, numerous water droplets adheredto the bag, and white mould was growing on the grapes (ComparativeExample 4). Moreover, while the bunch which had been packed in the filmbag from Composition No. 8 essentially looked good, the grapes had lostsome of their flavour (slight smell of alcohol) (Comparative Example 5).In contrast, in the case of the bunch which had been packed in the filmbag from Composition No. 2, the stems still looked fresh and green, andthe flavour of the grapes was excellent (Example 7).

EXAMPLE 8 Comparative Example 6

Film having a composition of A/B/C=65/25/10 by weight was produced inthe same way as in Example 1, and the properties are shown in Table 2.It can be seen that, even though samples of film may have about the samemoisture transmission, film based on polyethylene of high density hasinferior gas permeability.

TABLE 2 Composition Moisture CO₂ Gas Oxygen Gas Number A B CTransmission Permeability Permeability Example 8 Composition 9 A-2 B-2C-1 780 54,900 7,000 Comp. Ex. 6 Composition A-2 B-6 C-1 800 43,0005,000 10

EXAMPLE 9 Comparative Example 7

The lid was removed from a corrugated cardboard box for holding 22peaches, and then 22 peaches introduced immediately after they had beenpicked from the tree. The entire pack was then wrapped in either filmfrom Example 8 or nylon film, and heat-sealed. After keeping for 7 daysat room temperature (23° C.), the gas inside the pack was sampled andanalysed, after which the pack was broken open and the contentsexamined. The peaches which had been packed using the film from Example8 all still had a good appearance, there was nothing irregular when theinsides were cut open with a kitchen knife, and the taste was good.Further, the gas concentrations inside the pack were carbon dioxide=6%and oxygen=7% (Example 9). On the other hand, in regard to the peacheswhich had been packed using nylon film, three of the 22 peaches hadspoiled and darkened, and there was a considerable smell of alcohol. Thegas concentrations inside the pack were carbon dioxide=15% andoxygen=12%, and it was inferred that, with the gas permeability beinglow, there had been some anaerobic fermentation (Comparative Example 7).

EFFECTS OF THE INVENTION

As explained above, in the composition of the present invention, thereis blended, with a polymer which has polyether chains, a specifiedproportion of polyethylene or copolymer in which polyethylene is thechief component having a specified density and, in this way, it ispossible to obtain moulded articles with a desirable water vapourpermeability over a broad range while still maintaining a high gaspermeability and, in particular, it is possible to offer a packagingmaterial for the cultivation or storage of fruit and vegetables.

Although the invention has been described in conjunction with specificembodiments, it is evident that many alternatives and variations will beapparent to those skilled in the art in light of the foregoingdescription. Accordingly, the invention is intended to embrace all ofthe alternatives and variations that fall within the spirit and scope ofthe appended claims. The above references are hereby incorporated byreference.

What is claimed is:
 1. A thermoplastic resin composition comprising: (A)a polymer containing polyether chains as structural units, (B)polyethylene or a copolymer in which polyethylene is the substantialcomponent of a density specified by JIS K6760 of less than 0.91 g/cm³,and (C) a compatibilizing agent, blended in proportions by weight ofA/B/C=99.5 to 0.5/0.5 to 99.5/0 to 30 (where the total of A+B+C=100),wherein said composition has carbon dioxide gas permeability above about54,900 ml/m².day.atm and oxygen gas permeability above about 7,000ml/m².day.atm.
 2. A thermoplastic resin composition according to claim1, wherein the polymer (A) containing polyether chains as structuralunits is at least one member of the group consisting ofpolyether-polyamide block copolymers, polyether-polyester blockcopolymers and polyether-urethanes.
 3. A thermoplastic resin compositionaccording to claim 2, wherein the polyether chains are polyoxyethylenechains.
 4. A thermoplastic resin composition according to claim 1,wherein the density of (B) the polyethylene or copolymer in whichpolyethylene is the substantial component is less than 0.90 g/cm3.
 5. Athermoplastic resin composition according to claim 1, wherein (B) thepolyethylene or copolymer in which polyethylene is the substantialcomponent is produced by a single site catalyst.
 6. A thermoplasticresin composition according to claim 1, wherein the compatibilizingagent (C) is at least one member selected from the group consisting ofpolyolefins or the (meth)acrylate or vinyl acetate copolymers thereofwhich have been grafted or co-polymerized with unsaturated carboxylicacids, unsaturated carboxylic acid anhydrides or unsaturated epoxides.7. Molded articles formed from the composition of claim
 1. 8. Packagingmaterials comprising the molded articles of claim
 7. 9. Packagingmaterials according to claim 8, comprising molded articles for thecultivation or storage of fruit or vegetables.
 10. Packaging materialsaccording to claim 8, wherein the articles are film, sheet or bags.